Tetrazolium salts serve as substrates for active cellular dehydrogenases and reductases. In the presence of NADH/NADPH, these salts are reduced to formazan products and produce strong, distinct colors.

Since these reactions take place in cells actively producing NAD(P)H, tetrazolium substrates have gained widespread use as a means of distinguishing living, metabolically active cells through staining and through cell viability assays.